I have two Western Digital WD Caviar Green WD15EADS (1,5TB with 5400) in RAID1.
The DNS-3232 is connected via gigabyte ethernet to my WDTV Live and streams DVDs (VOB and ISO) without any problems.
I've tested streaming some HD demo content (1080p), which also runs smoothly.
So I don't expect you to have any trouble with the WD20EADS (2 TB) in a gigabyte environment.